The present invention relates to a device for generating holographic reconstructions with light modulators, comprising:                At least one pixelated light modulator, which is illuminated by at least one light source,        A focussing optical element array, where each optical element is assigned to a group of encodable pixels of the light modulator, and where the optical elements image the light sources into an image plane downstream the light modulator so as to form light source images, and        A control unit, which is connected to the light modulator, and which computes with the help of programming means the holographic code for the pixelated encoding surface of the light modulator.        
The term ‘pixelated light modulator’ shall not necessarily be understood in the context of this invention as a modulator which comprises an arrangement of discretely controllable elements. It can also be a modulator with a continuous encoding surface, which is formally divided into discrete elements by the information to be displayed.
Further, the term ‘optical elements’ shall not necessarily be understood to be or to comprise conventional glass lenses, but it can be construed in a wider sense to be or to comprise refractive or diffractive optical elements which fulfill the same function.
A device for generating holographic reconstructions of representations, in particular three-dimensional scenes, is described in document WO 2006/119920 A1.
If information for example of a computer-generated hologram is stored on the pixelated light modulator, and if the light modulator is illuminated with sufficiently coherent light, a reconstruction of a three-dimensional scene will be generated in a reconstruction space. However, undesired periodic continuations also occur in the form of higher diffraction orders, because of the discrete representation of the hologram in the light modulator. Depending on the hologram encoding method employed, undesired regions can also occur within a diffraction order, which must therefore be filtered.
A conventional method for eliminating disturbing diffraction orders is to use a filter unit, e.g. a 4f arrangement, which can filter such diffraction orders. The filter unit can be dimensioned such that it only lets pass regions which are smaller than or identical to one diffraction order.
Such a method is applied for example in document DE 10 2005 023 743 A1. This document describes a holographic projection device and a method for generating holographic reconstructions of scenes using one-dimensionally and two-dimensionally encodable light modulators, said device comprising a light source, a focussing optical system, the corresponding light modulator, a projection system, and a filtering aperture, which is arranged between the light modulator and the projection system, and which lies in the image plane of the light source image.
The focussing optical system represents for the light modulator an optical illumination system, and for the light source an optical imaging system which images the light source into the image plane of the optical illumination system, where the Fourier transform of the light modulator is simultaneously generated in the image plane of the light source.
The projection device comprises a control unit which does not only encode the light modulator dynamically, but which also tracks the visibility region and thus the holographic reconstruction to a changing observer position. To achieve this, a position detection system is provided, which is connected to the control unit. The code on the light modulator is modified such that the reconstruction of the three-dimensional scene appears in a horizontal, vertical and/or axial position horizontally and/or vertically displaced and/or turned by an angle, according to the actual observer position.
In a dimensioned modification of the size relations of the above-described projection device in the form of a large, observer-friendly direct-view device, e.g. with a display with a diagonal of 20 inches, which is the size of a typical desktop monitor, a filtering process is performed on the light modulator, where a single light source is provided for the coherent illumination of the entire light modulator in conjunction with a filter unit. The direct-view device with the 20-inches display can comprise the light source, a focussing optical system, the corresponding light modulator, a projection system, and a filtering aperture, which is arranged between the light modulator and the projection system, and which lies in the image plane of the light source image. The filtering aperture comprises an opening which only lets pass the desired one diffraction order of the Fourier transform of the light modulator. The projection system images the aperture into another plane, which represents the observer plane at the same time. The observer in the observer plane can see the holographic reconstruction in a visibility region which corresponds to one diffraction order of the Fourier spectrum.
The corresponding filter unit requires in addition to the filtering aperture at least two lenses of which at least one is about as large as the light modulator that represents the display. This means for example in the case of the holographically encoded 20-inches display panel that one lens must have a diameter of at least 40 centimeters.
Because lenses typically only exhibit an adequate image quality at a ratio of focal length and aperture of much larger than one, and because the filtering takes place at the position of the light source image, here in the focal plane of the first lens, a filter unit first wide lens, filtering aperture, second wide lens—which has a depth of substantially larger than 40 centimeters in front of the light modulator panel is required in this example. In the direct-view device with a light modulator panel as a screen, if a large display is used (e.g. with a diagonal of 20 inches), it is rather complicated to provide a wide lens which has about the size of the screen, where, in addition, the filter unit has a very large depth, as described.
One problem is that the design of a holographic direct-view device with the described dimensions of the optical components is very voluminous and heavy, which is undesired.
A further problem is that, in display holography, because of the pixel dimensions of commercially available light modulators, only very small useable diffraction angles are provided, which, in turn, cause a small observer window.
According to a method of display holography described in document U.S. Pat. No. 3,633,989, HPO (horizontal-parallax-only) holograms are used, where a hologram encoding is only performed in one dimension. Values for the one-dimensional hologram are computed independently of each other and are typically written to individual rows of a light modulator. In order to increase the diffraction angle, hologram values, which are typically encoded in multiple pixels arranged side by side, can in this case be encoded in pixels which are arranged below one another in multiple rows.
When using one-dimensional holographic codes within the light modulator, it will only be possible for a one-dimensional holographic reconstruction to take place. The light wave diffracted by the one-dimensional HPO hologram of the light modulator accordingly extends in the horizontal direction in the visibility region.